The present invention relates broadly to an optical system, and in particular to a universal wavefront sensor apparatus.
A wavefront sensor is needed to provide image control for large active optical systems in space. If the optical system is continuously pointing at the ground, it is appropriate to use ground imagery as a means to sense wavefront errors which are introduced by the optics within the optical system. Furthermore, since active optics image control is both a day and night need, the use of infrared imagery is also appropriate.
Therefore, it is clear that there exists a need for an infrared wavefront sensor which will be applicable to large optical systems that are operated in space. The required wavefront sensor should work on cloud detail or ground scene imagery, and use appropriate components so that it can operate in the infrared region and thereby be functional at all times.
The optical systems which are being considered for use in space, are extremely large. These optical systems cannot be expected to stay in focus or even in alignment without some type of sensor to determine the configuration errors and the mechanical adjustments which will be required to remove them. It has been shown that thermal expansion terms may necessitate system readjustment, and sometimes it is necessary to adjust as often as every 10 minutes, to prevent significant quality deterioration. In addition, the warping of structural materials with age could require system readjustment even if there were no temperature effects.
In the prior art, two radically different techniques for controlling image configuration errors in large optical systems may be utilized:
1. Measure absolute distances to control points to maintain exact configuration,
2. Use a wavefront sensor to determine wavefront shape.
The first technique relies on complete internal control of the optical system, while the second scheme relies on the imagery which is produced by the optical system and on the knowledge of the design characteristics of the optical system. An available optical position sensor (OPS), which can measure absolute distance, has been demonstrated to work well over a 10 meter path in a straight-through configuration. However, the following formidable problems yet remain to be solved. These problems are: (1) making a three-headed triangulation configuration, (2) providing precise beam aiming over a large solid angle, and (3) providing optical retro devices that are reliable in position to submicron tolerances which would be required for segmented mirrors.
The use of a wavefront sensor that can determine wavefront errors and is capable of using 12th magnitude stars for measurement has been demonstrated. It can theoretically work with 14th- magnitude stars when a two-stage light intensifier is supplied. This sensor, in combination with edge sensors for segmented mirrors, can control all significant errors in the optical system when star fields are available as sources. When star sources are not available, the wavefront sensor does not work.
Since a wavefront sensor is key to system quality control, and since narrow angle systems pointing at the ground do not have star fields available, it appears appropriate to look at wavefront sensor schemes that could work on scene imagery.
Another real scene device proposed analyzes intensity distributions in a scene image and by trial and error attempts to find wavefront errors that have affected the image. The process is extremely slow, requiring long integration times to produce a good signal-to-noise ratio in the data. The computational burden is extreme, requiring approximately 1 hour of a dedicated PDP 1170 computer for solution. The method is applicable only to long staring systems because of data gathering time. One other method (application Ser. No. 590,612, now abandoned) has been developed for finding the wavefront from extended daytime scenes. It works well but only during daylight hours.
The present invention uses principles much the same as those used by the daytime scene sensor. The sensor can work on cloud detail or ground infrared scene imagery, so that it can work well at all times. The measurement precision as limited by noise is calculated to be 0.03- micron wavefront error for a 0.1-sec sampling time. The wavefront sensor will operate in a variety of adverse conditions such as, on extended scene detail, daytime or nighttime or on cloud detail. The wavefront sensor apparatus may be utilized in large, satellite borne optical systems for control of system errors.